In most areas, the publicly-switched telephone network, or “PSTN”, is operable to provide the recipient of a telephone call with caller identification data related to the incoming call. This service is normally an option provided to the customers of a local exchange carrier, or “LEC” and is delivered over the PSTN telephone line to the customer's premises, referred to as the “local loop”.
Incoming call handshaking begins with a ring signal sent to customer premises over the PSTN local loop. In the United States, the ring signal is generally a 90-volt alternating current signal transmitted at 20 hertz, transmitted in a two second pulse every six seconds. The caller identification data is generally sent over the PSTN local loop between the first and second ring signal using a frequency-shift keying technique at 1,200 bits per second (1,200 baud). PSTN caller identification data generally includes the telephone number originating the call and a time and date stamp reflecting the current time. The data also commonly includes an ascii string of alphanumeric characters providing additional information regarding the call source.
A caller identification decoder attached to the PSTN local loop, which may be incorporated into a telephone or may be a separate unit, can decode the caller identification signal and provide the customer with information regarding the source of the incoming call. Generally, the decoder will incorporate some degree of memory for storage of caller identification information related to prior incoming calls in a call log. Accordingly, a customer who has been away from the phone for some time can review the call log and determine what phone numbers called that number and at what time.
Similar caller identification functionality may be provided by other telecommunications networks, such as cellular telephone networks, although the information provided over such networks is generally more limited than the caller identification information provided over the PSTN local loop. For example, it is known that not all caller identification signals provide complete time and date data for incoming calls. Accordingly, the caller identification data provided by non-PSTN sources cannot be relied upon for the generation of an accurate incoming call log.